Long QT syndrome (LQTS) is characterized by repolarization abnormalities and sudden death. Chromosome 7-linked LQTS results from mutations in HERG, a gene that encodes subunits of the human IKr potassium channel. Human IKr is modulated by extracellular potassium. The aims of this study are to determine if elevated potassium levels can be safely and reliably maintained in children with chromosome 7-linked LQTS for a period of one month, and to determine if this increase in serum potassium level results in normalization of the repolarization abnormalities noted on surface electrocardiograms. If the first two aims are met, then the study will be extended to see if the effect can be sustained by administering potassium for longer periods. Five children from four different families have participated thus far. Increasing serum potassium levels by supplementing potassium intake and administering oral spironolactone has resulted in a reduction in the QTc interval compared to baseline. No subject has developed symptomatic hyperkalemia, nor has a potassium level been measured greater than 5.9 mmol/l. No side effects have been noted. A dose-escalation phase of the study is now being initiated. This study, which is the pediatric arm of a similar study in adults, strongly suggests that patients with chromosome 7-linked LQTS will benefit from prolonged potassium supplementation, and this is likely to reduce the incidence of lethal arrhythmias.